"The Defense Intelligence Agency currently estimates Russia has a stockpile of 2,000 "non-strategic" nuclear weapons including short-range ballistic missiles, gravity bombs and depth charges that can go on medium range bomber aircraft," according to the report.

"DIA also estimates Russia has nuclear armed anti-ship, anti-submarine missiles and torpedoes. What do they need nuclear depth charges for?" one US official asked.

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President Donald Trump highlighted the importance of the review's conclusions Friday in a written statement.

"Over the past decade, despite United States efforts to reduce the roles and numbers of nuclear weapons, other nuclear nations grew their stockpiles, increased the prominence of nuclear weapons in their security strategies, and -- in some cases -- pursued the development of new nuclear capabilities to threaten other nations," Trump said.

"The strategy develops capabilities aimed at making use of nuclear weapons less likely. It enhances deterrence of strategic attacks against our nation, and our allies and partners, that may not come in the form of nuclear weapons. And, importantly, it reaffirms our commitment to arms control and nuclear non-proliferation, maintains the moratorium on nuclear testing, and commits to improving efforts to prevent, detect, and respond to nuclear terrorism," he added.

The Pentagon is adamant the Nuclear Posture Review walks the line between maintaining a nuclear deterrence and encouraging controls on nuclear weapons.

"It reaffirms that the fundamental role of US nuclear policy is deterrence and continues our clear commitment to nonproliferation and arms control," said Deputy Defense Secretary Patrick Shanahan.

First mention of Russian nuclear torpedo system

The report also publicly acknowledges, for the first time, that Russia is "developing" a "new intercontinental, nuclear armed, nuclear-powered, undersea autonomous torpedo."

Known in English as the "Status-6" system, the program is described by US officials as essentially a drone-type device fired underwater that can potentially travel thousands of miles and strike US coastal targets such as military bases or cities.

Upon detonation, the device is designed to cause large zones of radioactive contamination.

Some analysts have called it a "doomsday weapon," and US Sen. Tom Cotton, R-Arkansas, has labeled the concept "destabilizing."

"The concept is a horror of the Cold War," according to Adam Mount, a senior fellow and the director of the Defense Posture Project at the Federation of American Scientists. "It is clearly inspired by overblown Russian worries that US missile defenses will make their missile forces obsolete."

"There is no indication from public information that Russia is actively developing the system, but it is alarming to see it in a Pentagon document," Mount said, adding that while the program is referenced in the Nuclear Posture Review, it is not mentioned in other government reports.

Low yield nuclear weapons

The review calls for more focus on US "low yield" nuclear weapons to try to convince Russia that the US has a credible deterrent against the potential Russian threat. The plan calls for modifying existing US warheads on submarine-launched ballistic missiles in a $50 million five-year program.

Each submarine would only carry a few of these new missiles, armed primarily with strategic longer-range missiles.

But according to some experts, the plan translates Trump's urge to "greatly expand and strengthen" the arsenal into policy.

"The Trump administration's call for new nuclear weapons is a major shift in US policy," Mount said when asked the proposal for low-yield nuclear weapons.

"Programs for new nuclear weapons would follow our adversaries into a world where nuclear competition is commonplace. The programs would crowd out other military priorities, alarm allies, and have huge diplomatic cost -- for minimal deterrence benefit," he said.

Mount said that the review's outline for low-yield nuclear weapons "relies on the assumption that Russia would invade NATO allies, and glosses over important arguments about where and why these weapons would ever be necessary."

Low-yield ballistic missiles could be fielded within just a few years.

Longer-range missiles could come over the next decade, and the US would plan to develop and field sea-launched cruise missiles also with lower-yield warheads.

The review is calling for all of this along with an overall modernization of the nuclear force because the Pentagon requires an "investment in a credible nuclear deterrent with diverse capabilities," chief Pentagon spokesperson Dana White told reporters.

Defense Secretary James Mattis said: "What we have is a nuclear deterrent, so keep those two words always together and then look at the efforts to push forward on nonproliferation and arms control, and you have to do that when you're in a position of persuasion not of hope."